Blade Design, Manufacture, and Testing - Small Wind Turbines: Analysis, Design, and Application - page 141

the wall behind the blade. Monitoring these gauges gives a good record of the
development of fatigue damage because any change in the blade properties due to
fatigue will alter the strain.
Previous versions of the blade were mounted in the plane of rotation (or hori-
zontally in the rig shown) in much the same way as the smaller blades in Fig. 7.7 are
mounted in the turbine shown in Fig. 6.1 . However, the stress concentrations in the
transition region between the attachment and aerodynamic portion of the blade
induced delamination and fibre pullout on the tension (lower) surface after the
equivalent of only 7 years. Straightening the blade achieved superior strength with
fewer fibreglass layers at the cost of complicating the blade attachment. It is not
clear from the photograph that the blade holder is held by two load cells to monitor
the force and moment on the blade. This is not strictly necessary but it provides
information about the relationship between constant amplitude testing, which is
done for convenience, and constant force testing, which is likely to be more realistic.
The fatigue test program consists of a number of cycles at various frequencies
and stroke. Some of the ''bins'' of cycles are amalgamated if their parameters are
similar, and the tests are done partly in multi-year groups of single cycles. Both
these simplifications reduce the number of manual adjustments to the stroke. After
the photograph in Fig. 7.14 was taken, the blade survived undamaged an accel-
erated test program simulating 20 years of operation.
Fatigue behaviour of composite wind turbine blades is a complex subject,
e.g. [ 15 ], and is probably deserving of a topic on its own. Some basic and very
useful information on the fatigue behaviour of composites and timber is given in
Annex E of IEC 61400-2 which should be consulted by all blade designers. It is
likely that fatigue testing will become compulsory under future revisions of the
IEC standard and this will be accompanied by an increased knowledge of the
damage mechanisms and techniques to design against these. In the meantime it is
important to be careful and conservative in assessing small blade fatigue.
7.7 Forming the Rotor
Once blades have been made and tested for suitability and integrity, and the blade
holder designed and manufactured, the blades can be attached to form a rotor.
This process is usually straightforward, especially for small turbines without
pitch control, but there are a few issues that must be considered. The main one is
to decide on the maximum allowable imbalance of the rotor which is likely to
be the largest imbalance in the whole turbine and a potential source of vibration
and fatigue. IEC 61400-2 assumes a default eccentricity in the rotor centre of
mass, e, of 0.005R for the SLM as explained in Chap. 9 . For a two-bladed rotor,
e is given by